Nowadays, fiber to the home (hereinafter referred to as FTTH) service is available in which optical fibers are extended to individual homes to be used for exchange of various information.
As one form of FTTH that transmits various information, there is a system in which a broadcast signal and another communication signal are simultaneously transmitted in different systems by means of a single optical fiber (ITU-T Recommendation G.652). Generally in this system, the broadcast signal is often an analog signal or a baseband signal.
The characteristics of the system having an optical fiber as a transmission medium are as follows:
FTTH is typically a double-star type PON (Passive Optical Network), and has a large distribution loss (typically, up to 32 branches are assumed).
Since FTTH transmits an analog signal or a baseband signal, a CNR (Carrier to Noise Ratio) in the receiver is required to be high, and the required minimum signal light power in the light receiving portion is larger compared with the case of digital transmission used for communication.
In the system described above, the signal light power in the signal input portion needs to be large. Especially in consideration of attenuation and distribution loss during transmission of a signal light, higher power is required in a line with a longer distance or more branches. If a signal can be transmitted as far as possible and distributed to many subscribers at a time, it is more advantageous from various points of view (construction costs, maintainability, system design, etc.).
However, in an optical transmission using an optical fiber, even if light with more than a certain power is intended to be injected into an optical fiber, SBS, which is one type of non-linear phenomenon, allows the entrance of light with a certain amount of power (hereinafter, referred to as SBS threshold power) or less and the rejected light is returned to the entrance light side as backscattered light. This phenomenon sometimes puts restrictions on signal light power in the input portion, thus posing a problem (for example, see Non-Patent Document 1).
Conventionally, as methods for achieving SBS suppression, techniques for modifying optical characteristics in the longitudinal direction, the dopant concentrations, and the residual stress have been reported (see, for example, Patent Document 1 and Non-Patent Document 2).    Non-Patent Document 1: A. R. Charaplyvy, J. Lightwave Technol., vol. 8, pp. 1548-1557 (1990)    Patent Document 1: U.S. Pat. No. 5,267,339    Non-Patent Document 2: K. Shiraki, et al., J. Lightwave Technol., vol. 14, pp. 50-57 (1996)